In marine seismic exploration, a source of acoustic energy is released into the water every few seconds to produce appropriate acoustic waves that propagate from the source into the earth's surface. These acoustic waves, upon contacting the marine floor and subfloor geologic formations, are reflected back to recording instruments having transducers which convert these waves into electrical signals which are then recorded. Analysis of these electronic signals provides insight into the structure of the subsurface geological formations.
There have been many devices utilized for generating these seismic or acoustic waves. Most recently, however, a major marine seismic energy source has been the air gun. (The term "air gun" is intended to encompass an apparatus for dispersing any suitable compressible gaseous fluid such as air, steam, nitrogen, carbon dioxide, gaseous products of combustion and so forth.) These air guns are capable of releasing high pressure air on the order of 2,000 psi to 6,000 psi in the water to create the desired acoustic waves.
The acoustic pulse generated by an air gun is proportional to the bubble velocity formed by the air escaping the air gun. As air exits the gun ports a bubble is formed. This bubble accelerates outwardly generating the acoustic pressure pulse which creates the seismic wave. As long as the air bubble/water interface is accelerating, the acoustic pulse amplitude is increasing. Once the maximum bubble velocity is reached, then the maximum acoustic pulse is achieved.
A typical air gun includes an annular housing having a primary chamber in which compressed air is stored and exhaust ports which allow the stored air to escape from the housing. These guns also include a constant source of compressed air through an inlet passage in the housing which supplies the compressed air for the primary chamber and which enters a control chamber to force the shuttle into a closed position over the exhaust ports in the housing. A solenoid valve is used to allow air to flow into a firing chamber opposite the control chamber having a shuttle bearing surface of greater surface are than the bearing surface in the control chamber. This creates unequal pressure on the shuttle, forcing the shuttle to an open position to expose the exhaust ports and allow the compressed air to escape into the surrounding water. When the shuttle is in a prefire or closed position, the air gun is charged and ready for firing. When fired, by activating the solenoid, the compressed air escapes into the water.
It is well known that by employing a plurality of air guns in an array the resulting pulse "signature" generated by the simultaneous firing thereof improves the high frequency response of such signature. However the timing repeatability, or in other words the ability to repetitively fire the air gun array such that the compressed gases from each air gun simultaneous accelerate outwardly from the opened shuttle, is paramount when air gun arrays are used. Because the seismic wave pulse is a narrow pulse with a sharp peak, any misalignment in the release of compressed gases from the air gun can degrade the pulse signature.
Generally, in the type of air gun describe above, timing repeatability is dependent on the gun pressure, shuttle mass, air flow passages, solenoid valve air flow, and friction between the shuttle and the wear rings and seals. Of these, shuttle friction between the moving shuttle and wear rings and seals is thought to have the greatest influence on the opening velocity of the shuttle.
Attempts at reducing friction between these components have included the placement of fluid lubricants between the moving surfaces. Lubricants placement is achieved by the injecting a lubricant into the high pressure air supply. However, the high oxygen concentration in the pressurized air supply and the presence of a lubricant therein creates a potentially explosive and hazardous condition.
Additionally, elastic O-ring check valves overlying vent openings have also been used in attempts to block debris, such as suspended inorganic material, slit, marine life and the like, from entering and fowling the air gun. For example in operation, as compressed gases in the firing chamber enter the firing chamber vent, the force exerted by the tensioned O-ring is overcome thus causing the O-ring to expand outwardly and away from the firing chamber vent opening allowing these gases to exhaust into the water. Substantially simultaneous with the venting of the firing chamber gases, a much larger volume of compressed gases stored in the primary chamber exits the air gun. During this time of multiple gas discharge, debris in the water may be forced passed the expanded O-ring into the air gun and may ultimately fowl shuttle movement.
Thus, safer and more reliable means for reducing friction between critical air gun components and preventing debris from entering and fowling shuttle movement is needed.